Recursive enumerable languages

Recursive enumerable languages are a class of formal languages in computer science, particularly in the theory of computation. They consist of all the languages for which there exists a computer program (or algorithm) that can list all valid strings in the language. However, if a string is not part of the language, the program may run indefinitely without providing a clear answer. Essentially, these languages can be recognized by a computer, but they may not always provide a definitive outcome when tested for membership, making them more complex than recursive languages, which always have a clear yes or no answer.